BT currently uses several different systems in order to carry voice traffic and adsl broadband traffic.

Telephony or PSTN (Public Switched Telephone Network) itself can use several different systems in the local exchange such as System X or System Y. SystemX was first developed in 1979 and SystemY has been in use by BT since the mid 1980s. Most BT exchanges are System X.

ADSL Broadband Traffic is routed over BT's core network. The exchange backhaul uses BT's ATM network (MSiP) which was originally built in the early 90's for leased lines. From here it joins up to the Colossus IP Network which is the main UK backbone that has several access points and was built in the late 90's.

21CN stands for 21st Century Network and involves a major upgrade and unification of BT's core network.

Over the next few years BT will be rolling out 21CN across the UK eventually replacing the existing PSTN and ADSL Broadband equipment in the exchange as well as the core network with newer & more modern technologies.

This new Next Generation Network (NGN) will provide multi-services such as voice, video, content and data, as well as higher speed broadband, all carried over an IP based network. This new IP based network will eventually consolodate BTs existing 17 separate platforms.

BT expects to invest up to £10 billion in delivering the 21st century network programme over the next five years.*

~ Why do we need 21CN?

Technology has advanced and moved on since the original network was installed and many of the newer technologies have been bolted on to the existing (20CN) system meaning a lot of manual intervention.

Not only has the way we use technology to communicate changed, but we now also expect more media content to be streamed into our homes, than what we did even just a few years ago.

Much of the existing network and exchange equipment needs to be updated in order to take advantage of the higher speeds available on adsl2+ broadband.

It makes sense for BT to unify their existing networks replacing it with a single IP based system, which will be cheaper to run and more efficient to maintain.

~ When does my exchange go 21CN live?

Much of the core network is already in place, but upgrading the equipment in all the exchanges throughout the UK will take several years.

The upgrade process is in two stages - PSTN and ADSL. Most exchanges will likely see the broadband element go live before PSTN, but there will some exchanges that may see PSTN 21CN before the adsl element.

BT is aiming for for 55% broadband coverage by March 2010, with the intention of 75% coverage by Spring 2011.

You can check if your exchange is 21CN enabled, or when it is due to go live, by putting your details into the broadband exchange checker.

~ Is my ISP offering 21CN products?

Whether you can get 21CN products will depend on if your exchange is 21CN enabled, and if your ISP is offering 21CN services.

At the time of writing the following ISPs offer adsl2+ via 21CN or are in the process of doing so:-

Some of the larger ISP have their own equipment in local exchanges and may be offering adsl2+ via LLU.

~ What is adsl 2+?

Traditional adsl (G.992.1) only allows downstream speeds of up to 8Mbps.

ADSL 2+ (G.992.5) is able to make use of the previously unused higher frequencies on your telephone line to be able to transmit more data and get faster speeds of up to 24Mbps* downstream and up to 1.3 Mbps* upstream.

Annex M which allows upstream speeds of up to 2.5Mbps will also be deployed at a future date.

This will depend on your distance from the exchange. 21CN is primarily about upgrading BT's core network and equipment in the exchanges to be able to provide faster broadband to those users whose telephone line will support it.

ADSL2+ speeds do not depend on where abouts you live in the UK, but rather how far from the local telephone exchange you live, so its the quality and length of your telephone that is the important factor.
In most cases you should see some improvement on adsl2+.
The nearer you live to the exchange, the greater the speed increase should be.

To get a rough idea you can either use the adsl checker which may give you an estimate from the BT database if your exchange is live.
Alternatively this site's Maximum Speed Calculator should give you a better estimation of what your maximum speed is likely to be, based on your line stats.

~ Will I need any new equipment for 21CN?

21CN does not affect the local loop or home equipment. As far as the End User is concerned you do not need to do anything as regards to changing telephony equipment or adsl filters etc and change over should be relatively seamless consisting of just several minutes of down time whilst jumpering at the exchange is re-configured. You should be notified in advance of this occurring.

However if you have an older type modem or router, you may need a new adsl2+ compatible router to take advantage of faster speeds of up to 20Mbps available on adsl2+. ADSL 1 routers will still work on adsl2+ based systems as the new MSANs are backwards compatible, you just will not be able to get the higher speeds. Most modern routers are adsl2+ compatible.